SHA256 uses ForceZero now too

[JacobBarthelmeh]

Building using the following configure :

 ./configure --disable-filesystem --enable-singlethreaded --disable-examples --disable-crypttests --disable-benchmark --enable-static CPPFLAGS='-DNO_ERROR_STRINGS -DIGNORE_NAME_CONSTRAINTS -DNO_ASN_TIME -DUSE_SLOW_SHA -DUSE_SLOW_SHA256 -Os -ffunction-sections -fdata-sections' --disable-chacha --disable-poly1305 --disable-dh --disable-des3 --disable-sha512 --disable-rsa --disable-ecc --disable-sha3 --disable-sha224 --disable-md5 --disable-sha --disable-sha384 --disable-rng --disable-sp --disable-fastmath --disable-heapmath --disable-aesctr --disable-aesgcm --disable-pkcs12 --disable-aes --disable-asn --disable-kdf --disable-sp-math-all --disable-harden --disable-asm --enable-cryptonly

Before this change results in :

 nm src/.libs/libwolfssl.a | grep ForceZero
                 U ForceZero
                 U ForceZero

[JacobBarthelmeh]

Retest this please Jenkins